To containers or products formed of plastics such as PET, or glass or metal, stuck are various types of labels formed of plastic films or paper.
For attaching a label of a plastic film to a container, there are known a method of fitting a cylindrical thermoshrinkable film label and thermally shrinking it (shrink method); and a method of fitting a cylindrical stretchable film label while stretching it (stretch method) (Patent References 1 to 3). For attaching a label of paper, there are known a method of applying a paste to the back side of a label just before stuck to a container, and then drying the paste for sticking the label (glue paste method); a method of previously applying an adhesive paste (pressure-sensitive adhesive) to the back side of a label, and sticking it under pressure (tack paste method); and a method of previously applying a delayed paste capable of expressing its adhesiveness when heated, to the back side of a label, and heating the label with hot air just before stuck to thereby make the paste sticky, and thereafter sticking it under pressure (heat-seal paste method) (Patent References 4 to 7). However, these methods have various problems as mentioned below.
The shrink method and the stretch method require a step of forming a cylindrical label, and therefore the working steps increase and the fitting apparatus is large-scaled, and accordingly, these methods are problematic in point of the costs. In addition, they may be applicable to only labels of film material, and therefore, their application to containers for which labels of paper materials are liked, such as those for soy sauce, sake, Japanese sweets and the like, is limited in point of their design variation.
The glue paste method requires control of the coating amount of paste (water-based adhesive), paste exchange, and cleaning of used paste, therefore requiring operators' skill. In addition, the water-base adhesive tends to dry slowly when applied to others than paper labels, but on the contrary, it is hard after dried; and therefore, it is defective in that, when it is applied to paper labels, the substrate may be readily broken and, after peeled, the substrate may often remain on the container surface along with paste remaining thereon. However, different from the tack paste method, this does not require a release sheet, and therefore, it is a resource-saving one like the heat-seal paste method. Further, the method comprises only paste application and sticking, and therefore, its equipment may be simple; and its other advantages are that the label-sticking speed is high and the production costs may be reduced in point of the plant and equipment investment and the working efficiency.
In the tack paste method, a tack paste is previously applied, and therefore, the method requires a processed paper that is specifically so processed as not to adhere to others just before the label sticking (this is referred to as release paper). The labeled container itself may be used just after the label sticking thereto, but the release paper is discarded as trash, therefore resulting in the increase in trash, and this is problematic in point of the waste of resources. Further, the release paper is expensive, therefore resulting in the increase in the costs. In addition, since the speed at which the label is stuck to containers is slow, and the method is unfavorable for labels for containers that are mass-produced in large quantities.
In the heat-seal paste method, a delayed paste that is not adhesive before heated but becomes adhesive after heated is previously applied to the back side of a label, and in this, therefore, the label can be readily stuck to a container only by heating it to make the paste adhesive and by applying pressure to it. Differing from the tack paste method, this method does not require release paper, and therefore this may be free from the problem of waste of resources. However, this has a problem in that, when the label is heated too high when stuck to plastic containers, they may deform; and therefore, the label must be heated and stuck at a relatively low temperature. The delayed paste generally has a composition prepared by mixing a thermoplastic resin having a glass transition temperature of from 0 to 40° C. or so and a solid plasticizer and optionally a tackifier; but when the delayed paste-coated label roll is stored in a warehouse or transported in high-temperature environments, especially in the summer, or when it is used in a thermal printing system, then the label may become sticky before use to cause blocking, and this may be another problem of making trouble.
Patent Reference 1: JP-A 56-48941
Patent Reference 2: JP-UM-A 2-37837
Patent Reference 3: JP-A 1-99935
Patent Reference 4: JP-A 55-76378
Patent Reference 5: JP-A 5-173487
Patent Reference 6: JP-A 2004-29656
Patent Reference 7: JP-A 5-4297